This invention relates to a record carrier body serving as an optical type information recording medium and, more particularly, to a record carrier body for storing information therein by forming its recording layer with protuberances instead of holes or pits.
It is well known in the art to project a laser beam (which has been pulse-modulated according to write information) onto a recording layer of an optical type record carrier body such as an optical disk to thereby form an arrangement of pits corresponding to the write information in the recording layer. In this type of data recording technique, however, it is inevitable that a so-called "rim" of the material of the recording layer will be undesirably formed surrounding each pit in the area in which the laser beam is incident and/or that the recording layer material will be thermally fused and sputtered. Such rims of sputtering of the recording layer material will cause noise, when the information stored in the optical disk, in the above manner, is read out in playback. The signal-to-noise ratio of the reproduced data is thereby reduced. Moreover, the formation of pits in the recording layer of the optical disk requires heating the recording layer to a temperature above the melting or boiling point of the layer with the laser beam. This heating deteriorates the quality of the recording layer itself and reduces the life of the optical disk.
Optical disks, in which the recording layer is adapted to be formed with protuberances or plastically raised portions, rather than pits, when writing information, have been developed. Japanese Patent Disclosures (KOKAI) Nos. 56-65341 and 56-127937 show optical disks of this type.
In the data recording media disclosed in Japanese Patent Disclosure (KOKAI) No. 56-127937, no pits are formed in the recording layer when recording information, so that the drawbacks mentioned above can be overcome. However, the recording sensitivity is inferior. To be more specific, the data recording medium disclosed includes an organic intermediate layer, which is formed on a substrate and does not absorb energy, and an energy-absorbing layer consisting of a metallic material formed on the intermediate layer. In an information write mode, the energy-absorbing layer is locally heated in areas struck by a laser beam. The locally heated portions of the energy-absorbing layer heat the organic intermediate layer. That is, the organic intermediate layer is heated indirectly by the laser beam striking the energy-absorbing layer. When the organic intermediate layer is heated, it generates gases to push out and plastically deform the upper energy-absorbing layer. The energy-absorbing layer is thus formed with protuberances corresponding to the recording information. The efficiency of information writing, therefore, is extremely poor, and a high energy laser beam is undesirably necessary for writing data.
The optical type information recording medium disclosed in Japanese Patent Disclosure (KOKAI) No. 56-65341, involves difficulties in manufacture. More specifically, the recording medium includes a metallic light-reflecting layer, a gas-liberating layer that does not absorb energy, and a metallic energy-absorbing layer, which are laminated in the mentioned order on a substrate. The recording of information is performed again by locally heating the uppermost energy-absorbing layer with a laser beam, whereby the intermediate layer is indirectly heated to generate gases to form protuberances of the uppermost layer in the manner as described above. The basic difference of this recording medium from that disclosed in the previously mentioned Japanese Patent Disclosure (KOKAI) No. 56-127937 is that the light-reflecting layer is sandwiched between the substrate and energy-absorbing layer to increase the efficiency of laser energy utilization. However, in order to improve the recording sensitivity of the recording medium of this type, it is necessary to accurately control the thickness of the light transmitting layers (i.e., the intermediate gas-liberating layer and energy-absorbing layer) according to the wavelength of the writing laser. This means that difficulties occur in the layer thickness control, in the manufacture of the recording medium. Moreover, because the recording medium has a three-layer structure, its manufacturing process becomes complicated, which also reduces the yield.